The increasing miniaturization of electrical devices together with the often conflicting requirements for high volume, mass production at low manufacturing and applied cost but with zero defect, place exacting demands on connectors and associated circuitry.
Attempts to satisfy such requirements are described in the above-mentioned parent application as well as in Japanese patent publication number 3-112082 published May, 1991 in which the receptacle connector 310 of a film connector comprises a housing 311 molded in one piece of insulating plastic material with a plug receiving cavity 311a opening to a mating face 311b, film circuit supporting members 312 and 313 respectively, in each of which flexible film circuits 314 and 315, respectively, are insert molded to form integral bodies retained inserted in the housing 311 by an elongate T-section base member 317. Prior to insertion, a pair of identical helical coil springs 316 are mounted in longitudinally extending through-slots or grooves in the film circuit supporting members 312, 313 behind film circuit contact areas 314a and 315a biassing then to protrude into the cavity 311a.
The plug connector 320 comprises a housing 321 having a receptacle connector receiving cavity 312a, first and second film circuit supporting and locating members 322 and 323, respectively, first and second film or sheet-form flexible circuits or boards 324 and 325, respectively, in-molded in respective film circuit supporting members to form integral bodies which are inserted in the housing and retained by a base member 327 of generally T-shaped cross-section. Film circuit contact areas 324a and 325a remain exposed during the in-molding process and are pressed into engagement with contact areas 314a and 314b, respectively, on mating the plug and receptacle connectors with resilient depression of the springs 316. Connection to respective circuit boards 301 ad 302 is effected with training portions of the film circuit which extend out of the respective housings adjacent the circuit boards by a reflow soldering technique.
A disadvantage of such approach is that the counter-force to the spring contact force acts directly on the plastic walls 311 of the receptacle housing tending to distort or distend the walls by bending with a risk of breakage and decrease or loss of contact force. Clearly, increasing the contact force to assure more reliable electrical connection increases the stress on the housing while any distortion of the housing walls, which may be progressive over time, e.g. creep, will reduce the contact force and reduce reliability.
Similar problems may arise with another prior receptacle connector described in Japanese patent publication number UM 56-59774 and acknowledged as prior art in the parent application with reference to FIGS. 21 and 22 thereof, in which film circuits have leading ends secured around a cylindrical body of metal loaded elastomer located in a housing compartment adjacent the mating face and providing the contact force.
In a different approach, disclosed in Japanese Patent Publication 40-2588 and U.S. Pat. No. 3,154,356 to Crimmins, issued Oct. 27, 1964, reliance for contact pressure between engaging film circuits of matable parts and receptacle connectors is placed on inwardly projecting, curved, protuberances on end portions of walls of a spring which has been bent into channel or U-section within which the film circuit extends in slack condition. The spring is formed by bending a metal plate resulting in risk of an unevenness of the curvature of the protuberance along the length of the spring which may also be subject to relaxation while, the slack condition of the film circuit increases risk of shift and contact misalignment, particularly with conductive tracks at desirably small pitches. In addition, in that construction, the channel walls should be relatively long (high, increasing the size of the connector structure undesirably).
There is clearly a risk that an uneven spring contact force will result in faulty connection to some circuit paths.
Further problems may arise in using conventional film circuitry both on mating, when frictional engagement or butting together of the conductive tracks of the film circuit possibly causes peeling or abrasion thereof, or inadvertent cross connection at the surface of the circuit board arising from using the reflow solder process with conductors at very close pitch.
Other problems arise in securely anchoring the connectors to the circuit board with increased risk of accidental abutment or strain as a result of the high density of components on the circuit board and the consequently increased desirability of at least some degree of effective electrical shielding while occupying a minimum of circuit board area.